Mainly two types are hitherto known as methods of forming a direct-positive image. One of them is a method in which fog nuclei are previously formed in silver halide grains, and the fog nuclei are imagewise destroyed by solarization, Herschel effect or the like, and the remaining fog nuclei are developed to form a positive image. The other type is a method in which an internal latent image silver halide emulsion not previously fogged is used, which is subjected to fogging treatment (development nucleus formation treatment) after imagewise exposure and then surface development is carried out, or the surface development is carried out while applying fogging treatment (development nucleus formation treatment) after imagewise exposure, to form a positive image.
Of the two methods for the formation of positive image as described above, the latter type method can achieve in general a higher speed than the former type method, thus being suited to the application in which a high speed is required.
In the present technical field, various techniques have been hitherto known. For example, methods are known which are described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,762, 266, 3,761,276 and 3,796,577, and British Pat. No. 1,151,363.
The above methods for the fogging treatment (development nucleus formation treatment) may be carried out either by giving the whole surface exposure or by chemical procedures using a fogging agent, also by using a strong developing solution, or further by heating or the like.
Employment of these known techniques enables preparation of photographic light-sensitive materials that form positive images, but more improvement in photographic performance and solution of problems in manufacture have been sought in order to bring these photographic light-sensitive materials to be applicable in every photographic field.
In the present technical field, it is commonly known to prepare a direct-positive silver halide photographic light-sensitive material with use of two or more kinds of internal latent image emulsions. For example, U.S. Pat. No. 4,035,185 discloses that core/shell emulsions with varied degrees of internal chemical sensitization are mixed, but the emulsion in which the degree of internal chemical sensitization has been increased can have no sufficient efficiency for the formation of positive images, can also have no sufficient maximum density despite a mixed emulsion, and has been desired to achieve a still higher maximum density.
Japanese Patent Publications Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publications) Nos. 111938/1983 and 77436/1984 discloses the mixing and double laver coating of a core/shell emulsion and a fine grain emulsion. The fine grain emulsion, however, can not form an image when used alone, and is used merely to improve covering power, so that it can not enhance the photographic performances possessed by respective emulsions, also resulting in a higher minimum density.
Also, in the direct-positive silver halide photographic light-sensitive materials, developing is carried out in the presence of a fogging agent after imagewise exposure, thereby forming a fog nucleus in silver halide emulsion and carrying out the development, so that their photographic performances greatly depend on the conditions under which the emulsion is fogged, i.e., the type and amount of fogging agents or the character of developing solutions. Accordingly, in regard to, for example, the developing solutions, there is a demand for a direct-positive silver halide photographic light-sensitive material that has stable photographic performances against changes in composition or the lowering of developing ability owing to running or fatigue.
Moreover, in the direct-positive silver halide photographic light-sensitive materials, an attempt to reproduce with fidelity the lightness of objects results in requirement of a soft gradation performance having a broad exposure latitude. Hence, for example, the broad exposure latitude is obtained, or further the gradation is controlled, by mixing internal latent image silver halide grains having different grain size and substantially the same light-sensitive wavelength or by overlapping them by coating in different layers. In this occasion, however, because of employment of the grains having different grain size, the development proceeds in a different manner for each grain having different grain size, and for this reason the development proceeds slowly in respect of larger grains and the development proceeds quickly in respect of smaller grains, thus sometimes resulting in the so-called poorly continuous gradation that the gradation greatly differs at the toe and the shoulder of the characteristic curve thereof, particularly in the direct-positive color photographic light-sensitive materials, this point has not been sufficiently settled despite the importance of the balance in the toe portion of the characteristic curve. Satisfactory results are not necessarily obtained also in regard to the stability of direct-positive silver halide photographic light-sensitive materials, when the development conditions have been varied.
As a measure relating to this stability against the variation of developing conditions, some methods have been proposed in which emulsion grains are made development-active, e.g., the emulsion grains are made to have a small grain size, or a coupler capable of rapidly reacting with an oxidized developing agent to used, but these are still not only insufficient for obtaining always stable performances, but also accompanied with an increase in minimum density, undesirably.